By utilizing a domain inversion phenomenon of ferroelectric, it is possible to form periodic domain inverted regions (a domain inverted structure) inside the ferroelectric. Such domain inverted regions are utilized for a frequency modulator and an optical wavelength conversion element. Particularly, it is desired to realize an optical wavelength conversion element capable of making short wavelength and an optical wavelength conversion element for high-power output, said optical wavelength conversion elements using a ferroelectric having an excellent non-linear optical effect.
In order to expand the range of wavelength conversion of an optical wavelength conversion element (that is, make wavelength shorter), it is necessary to shorten the period of domain inverted regions. A conventional manufacturing method performs a proton exchange on the surface of a ferroelectric between periodic electrodes in order to form domain inverted regions of short period (see patent literature 1, for example)
On the other hand, in order to be capable of bearing a high-power output, it is necessary to make domain inverted regions have a high aspect ratio (depth/width) by increasing the thickness in a direction perpendicular to the direction of light incident on a periodically poled structure. As a ferroelectric suitable for manufacturing a periodically poled structure being thick in thickness, there are known substantially stoichiometric lithium niobate (LiNbO3; hereinafter referred to as SLN) and substantially stoichiometric lithium tantalate (LiTaO3; hereinafter referred to as SLT) (see non-patent literature 1, for example).
FIG. 11 is a diagram showing a method of manufacturing a periodic domain inverted region according to the conventional art. A device 1100 comprises a lithium niobate single crystal 1101, a comb-shaped electrode 1102, a flat electrode 1103 and a proton exchange region 1104. The proton exchange region 1104 refers to a domain where a proton exchange step is performed around the comb-shaped electrode 1102 and on the surface of the lithium niobate single crystal 1101-using the comb-shaped electrode 1102 as a mask. The ferroelectricity of the lithium niobate single crystal 1101 is deteriorated in the proton exchange region 1104.
A voltage is applied to such a device 1100, using a DC power source 1105 and a high-voltage pulse power source 1106. A voltage is applied to the lithium niobate single crystal 1101 between the comb-shaped electrode 1102 and the flat electrode 1103 to invert the polarization. Since the ferroelectricity of the lithium niobate single crystal 1101 in the proton exchange region 1104 is deteriorated, the sectional area of the generated domain inverted region is said not to become larger than the sectional area of the comb-shaped electrode 1102. And in such a way there is performed an operation of obtaining a domain inverted region of short-period.
Patent literature 1; Japanese Patent Laid-Open Publication No. 2000-147,584
Non-patent literature 1; Kitamura and Terabe, “Science & Technology Journal” October 2002, pp. 70-73